The present invention relates to an electric power steering apparatus which uses an electric motor as a generating source for steering assisting force.
Steering of automobiles is carried out in a manner that a rotating operation of a steering wheel located in a saloon is transmitted to a steering mechanism located outside the saloon in order to steer traveling wheels (front wheels, in general).
FIG. 1 is a cross sectional view showing a conventional electric power steering apparatus. As shown in FIG. 1, the electric power steering apparatus for automobiles includes an input shaft mechanism section HI, a torque sensor mechanism section H2 and a reduction mechanism section H3.
The input shaft mechanism section H1 has an input shaft 100 connected to a steering wheel 300. The torque sensor mechanism section H2 has a torque sensor 202 which detects a steering torque on the basis of a relative displacement of the input shaft 100 and an output shaft 201 connected to the input shaft 100 via a torsion bar 200. The reduction mechanism section H3 reduces a rotation of a steering auxiliary electric motor M (hereinafter, referred to simply as "motor M") driven on the basis of the detected result of the torque sensor 202 by means of a reduction mechanism 203, and transmits it to the output shaft 201. The rotation of the motor M helps an operation of a steering mechanism 400 in accordance with a rotation of a steering wheel 300 so that a driver's steering load can be reduced.
Further, in order to improve a collision safety performance, the input shaft 100 includes a first input shaft 101, a second input shaft 102, a shearing pin 103; 103 made of a synthetic resin, a wave form energy absorbing plate 106, and a third cylindrical input shaft 104. The first input shaft 101 has one end portion connected to the steering wheel 300 and a fitting cylindrical portion 105 at the other end portion thereof. The second input shaft 102 has one end portion which is fitted into the fitting cylindrical portion 105 so as to be slidable, and the other end portion connected to the torsion bar 200. The shearing pin 103; 103 is interposed between the first input shaft 101 and the second input shaft 102, and is ruptured by a longitudinal impact energy generated when a driver hits on the steering wheel 300 by a frontal (head-on) collision of automobiles. The energy absorbing plate 106 absorbs an impact energy by a relative movement of the first input shaft 101 to the second input shaft 102. The third input shaft 104 is connected to the other end portion of the second input shaft 102.
FIG. 2 is a view to explain a layout configuration of the conventional electric power steering apparatus. In the electric power steering apparatus constructed as described above, as shown in FIG. 2, the layout is made in a manner that the input shaft mechanism section H1, the torque sensor mechanism section H2, the reduction mechanism section H3, a steering joint section H4 provided between the reduction mechanism section H3 and the steering mechanism 400 and a pinion shaft section H5 are arranged in the above named order from the steering wheel 300 side. In other words, the layout is made in a manner that the torque sensor mechanism section H2 is arranged between the input shaft mechanism section H1 and the reduction mechanism section H3.
By the way, in the conventional electric power steering apparatus constructed as described above, in the case where an impact force is applied to the steering wheel 300 due to the frontal collision of automobiles, the shearing pin 103; 103 is raptured, and the first input shaft 101 slides in the longitudinal direction with respect to the second input shaft 102 so that the impact stress can be absorbed by means of the energy absorbing plate 106. In recent years, in order to further improve collision safety performance, it has been requested to make long an energy absorption stroke S (hereinafter, referred simply to as "stroke S").
In order to make long the stroke S, the entire length of the input shaft mechanism section H1 is made long, and further, there is a need of making long a relative slidable length of the first input shaft 101 and the second input shaft 102 in the case where the shearing pin 103; 103 is broken, that is, the aforesaid stroke S.
However, the conventional electric power steering apparatus is constructed in a manner that the input shaft mechanism section H1, the torque sensor mechanism section H2, the reduction mechanism section H3 and the steering joint section H4 are arranged in the named order from the steering wheel 300 side. For this reason, there is a limit in the layout to further make long the stroke S.
More specifically, the electric power steering apparatus has the layout configuration as shown in FIG. 2. However, in response to user's instruction and request, it is impossible to change a coordinate 01 which is a connecting point of the steering wheel 300 and the first input shaft 101, a coordinate 02 which is a connecting point of the output shaft 201 and an upper end of the steering joint, a coordinate 03 which is a connecting point of a lower end of the steering joint and a pinion shaft, and a coordinate 04 which is a center in engagement of a pinion of the pinion shaft and a rack. Further, in response to user's instruction and request, it is impossible to change a column mount length L between the coordinates 01 and 02 for attaching the input shaft mechanism section H1, the torque sensor mechanism section H2 and the reduction mechanism section H3 to a vehicle body, and to change a mount point of the motor M. For this reason, a longitudinal dimension is inevitably set in the input shaft mechanism section Hi, the torque sensor mechanism section H2 and the reduction mechanism section H3. Thus, in the layout configuration as shown in FIG. 2, in the case where the study is made so that the maximum stroke S is obtained in the limited longitudinal dimension of the input shaft mechanism section H1 taking the column mount length L and the mount point of the motor M instructed and requested by user into consideration, it is difficult to further make long the stroke S.